1. Field of the Invention
The present invention relates to a power transfer device for four-wheel drive in automotive vehicles, and more particularly to a power transfer device of the type in which a shift mechanism is arranged to temporarily provide a two-wheel drive mode.
2. Discussion of the Background
Disclosed in Japanese Utility Model Early Publication No. 61-109831 is a power transfer device which includes a center differential of the bevel gear type arranged to split the driving torque from a power transmission into two torque delivery paths respectively for front-wheel drive and rear-wheel drive, a front differential of the beve gear type arranged coaxially with the center differential to transfer the split driving torque from a first side gear of the center differential to front-wheel axles, and an output ring gear arranged coaxially with the center differential and in surrounding relationship with a hollow shaft extending from a second side gear of the center differential to transfer the split driving torque from the second side gear to rear-wheel axles through an output gearing assembly. In the transfer device, a clutch sleeve is axially slidably mounted on the hollow shaft for rotation therewith and shiftable between a first position where it is engaged with a mounting case of the output ring gear to provide a four-wheel drive mode and a second position where it is disengaged from the mounting case of the output ring gear to provide a two-wheel drive mode. A shift mechanism of the clutch sleeve includes a shift fork coupled with the clutch sleeve and carried on an axially slidable fork shaft, an operation mechanism of the fork shaft, and a detent mechanism arranged to retain the fork shaft in a shifted position.
In the case that the power transfer device is adapted to a full-time four-wheel drive vehicle, the clutch sleeve is shifted to the second position only when the vehicle is placed on a chassis dynamometer at its front or rear road wheels for measurement of emission gas, rate of fuel consumption or the like or when the vehicle is trailed in a condition where it is suspended from a tractor at its front or rear road wheels. In such adaptation of the power transfer device, utility value of the shift mechanism becomes less than in a part-time four-wheel drive vehicle. It is, therefore, desirable that the shift mechanism is composed of a small number component parts and manufactured possibly at a low cost.